The present invention relates to apparatus for uniformly heating relatively large area substrates for the glow discharge deposition thereonto of amorphous silicon alloy layers. More particularly, this invention contemplates the use of a plurality of heating elements for uniformly warming the substrate to an optimum deposition temperature. In the preferred embodiment, the heating elements are spacedly supported above the substrate by a generally A-shaped frame so as to extend from both of the lowermost sides to the raised central portion of said frame. In other words, the heating elements are angularly disposed by the frame so as to be at a minimum distance from the substrate adjacent the edges of the substrate and at a maximum distance from the substrate adjacent the center of the substrate. In this configuration, the edges of the substrate receive the most intense radiation from the heating elements which results in a uniform temperature distribution across the transverse width of the substrate. Should the substrate be used in deposition chambers wherein portions of the substrate other than its edges lose heat at the greatest rate, a uniform temperature distribution across the transverse width of the substrate may be obtained by varying any or all of the following parameters: the angle of the heating elements relative to the substrate, the distance which the heating elements overhang the edges of the substrate and/or the height above the substrate to which the heating elements are raised. As a matter of fact the intensity of radiation upon different parts of the substrate can be modified to achieve a variety of different temperature distributions across the transverse width of the substrate.
Recently, considerable efforts have been made to develop processes for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices which operate in a substantially equivalent manner to those produced by their crystalline counterparts. For many years such work with amorphous silicon or germanium films was substantially unproductive because of the presence therein of microvoids and dangling bonds which produce a high density of localized states of the energy gap. Initially, the reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein silane (SiH.sub.4) gas is passed through a reaction tube where the gas is decomposed by a radio frequency (r.f.) glow discharge and deposited on a substrate at a substrate temperature of about 500-600 degrees K. (227-327 degrees C). The material so deposited on the substrate is an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material, phosphine gas (PH.sub.3), for n-type conduction, or diborane (B.sub.2 H.sub.6) gas, for p-type conduction is premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The material so deposited includes supposedly substitutional phosphorus or boron dopants and is shown to be extrinsic and of n or p conduction type. The hydrogen in the silane was found to combine, at an optimum temperature, with many of the dangling bonds of the silicon during the glow discharge deposition to reduce the density of the localized states in the energy gap, thereby causing the amorphous material to more nearly approximate the corresponding crystalline material.
It is now possible to prepare greatly improved amorphous silicon alloys, that have significantly reduced concentrations of localized states in the energy gaps thereof, while providing high quality electronic properties by glow discharge. This technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
Activated fluorine readily diffuses into, and bonds to, amorphous silicon in a matrix body to substantially decrease the density of localized defect states therein. This is because the small size of the fluorine atoms enables them to be readily introduced into an amorphous silicon matrix. The fluorine bonds to the dangling bonds of the silicon and forms a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than could be formed by hydrogen, or other compensating or altering agents which were previously employed. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen, because of its exceedingly small size, high reactivity, specificity in chemical bonding, and having highest electronegativity.
Compensation may be achieved with fluorine, alone or in combination with hydrogen, upon the addition of such element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages, permitting the elements to form a silicon-hydrogen-fluorine alloy. Thus, alloying amounts of fluorine and hydrogen may, for example, be used in a range of 0.1 to 5 percent or greater. The alloy thus formed has a lower density of defect states in the energy gap than can be achieved by the mere neutralization of dangling bonds and similar defect states. In particular, it appears that use of larger amounts of fluorine participates substantially in effecting a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to the aforementioned characteristics, is an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. Fluorine, also influences the bonding of hydrogen by acting to decrease the density of the defect states which hydrogen normally contributes. The ionic role that fluorine plays in such an alloy is an important factor in terms of the nearest neighbor relationships.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage is increased without substantially decreasing the short circuit current.
Many publications on crystalline stacked cells following Jackson have been reported and, more recently, several articles dealing with Si--H materials in stacked cells have been published. Marfaing proposed utilizing silane deposited amorphous Si--Ge alloys in stacked cells, but did not report the feasibility of doing so. (Y. Marfaing, Proc. 2nd European) Communities Photovoltaic Solar Energy Conf., Berlin, West Germany, p. 287, (1979).
Hamakawa et al., reported the feasibility of utilizing Si--H in a configuration which will be defined herein as a cascade type multiple cell. The cascade cell is hereinafter referred to as a multiple cell without a separation or insulating layer therebetween. Each of the cells was made of an Si--material of the same band gap in a p-i-n junction configuration. Matching of the short circuit current (J.sub.sc) was attempted by increasing the thickness of the cells in the serial light path. As expected, the overall device Voc. increased and was proportional to the number of cells.
Due to the beneficial properties attained by the introduction of fluorine, amorphous alloys used to produce cascade type multiple cells now incorporate fluorine to reduce the density of localized states without impairing the electronic properties of the material. Further band gap adjusting element(s), such as germanium and carbon, can be activated and are added in vapor deposition, sputtering or glow discharge processes. The band gap is adjusted as required for specific device applications by introducing the necessary amounts of one or more of the adjusting elements into the deposited alloy cells in at least the photocurrent generation region thereof. Since the band gap adjusting element(s) has been tailored into the cells without adding substantial deleterious states, because of the influence of fluorine, the cell alloy maintains high electronic qualities and photoconductivity when the adjusting element(s) are added to tailor the device wavelength characteristics for a specific photoresponse application. The addition of hydrogen, either with fluorine or after deposition, can further enhance the fluorine compensated or altered alloy. The post deposition incorporation of hydrogen is advantageous when it is desired to utilize the higher deposition substrate temperatures allowed by fluorine.
It is of obvious commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over relatively large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method and Apparatus For Continuously Producing Tandem Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy. The deposition of each amorphous alloy within each of the dedicated chambers requires that the temperature of the substrate be elevated to approximately 250.degree.-300.degree. C. As with all other parameters of the deposition process, the temperature must be maintained close to the desired range across the transverse width of the substrate in order to produce a photovoltaic device of optimum efficiency. Variations in the temperature across the transverse width of the substrate within a deposition chamber can result in warped substrates which in turn can cause variations in quality of the amorphous layers deposited thereupon. Furthermore, the undulations of a warped substrate may cause the alloy layers deposited on the substrate to scrape against a wall of the passageway as it moves through the gas gate separating adjacent deposition chambers. It is therefore important that the temperature distribution along the transverse width of the substrate be maintained as close as possible to the selected uniform elevated temperature level.
Prior art heating elements essentially comprised a plurality of spaced, elongated quartz infrared light bulbs disposed generally horizontally across the entire transverse width of the substrate. This prior art disposition of single bulbs, the filaments of which extended across and was positioned substantially parallel to the transverse width of the substrate, resulted in the intensity of radiation directed from the filaments onto the substrate to be approximately twice as great at the center of the substrate than at the edges thereof. Further, since the substrate commonly rested upon or slidably engaged a supporting surface, the edges thereof acted as heat sinks which drew heat at the greatest rate from the substrate. It should therefore be apparent that the filaments of the heating elements must be disposed so as to direct the greatest degree of intensity of radiation adjacent to the edges of the substrate to compensate for the highest rate of heat loss occurring adjacent those edges.
Based on the foregoing, the apparatus of the present invention contemplates the use of two infrared light bulbs having a filament length of about eight (8) inches to replace each of the individual, horizontally disposed heating elements having a filament length of approximately sixteen (16) inches which were previously employed. The shortened length filaments, like the elongated filaments of the prior art, are adapted to extend substantially across the entire transverse width of the substrate. However, instead of disposing the shortened filaments of the present invention in a plane substantially parallel to the plane of the substrate, said shortened filaments are angularly disposed relative to the plane of the substrate such that the portions of the filaments adjacent the edges of the substrate are closest to the substrate and the portions of the filaments adjacent the center of the substrate are more remote from the substrate. More particularly, it has been determined that (1) disposing the filaments of the heating elements of the present invention at an angle of approximately 20 degrees relative to the plane of the substrate, and (2) having the ends of the filaments of the heating elements substantially overhang the edges of the substrate (by approximately one and one-half (11/2) inches), provides a uniform heat distribution across the entire transverse width of the substrate.
These and the many other objects and advantages of the present invention will become clear from the drawings, the detailed description of the invention and the claims which follow.